1. Atomic Structure History and Theories

2. The Greeks 4 th century B.C. Democritus “Atomists” school of thought Matter is composed of tiny indivisible particles called atoms.

3. John Dalton Dalton’s Atomic Theory All elements are composed of submicroscopic indivisible particles called atoms. Atoms of the same element are identical. Atoms of different elements are different. Atoms of different elements can physically mix together or chemically combine. Chemical reactions occur when atoms are separated, joined, or rearranged.

4. J.J. Thomson English physicist discovered electrons in 1897. Experimented with flow of electron current through gases. Electrodes connected to high energy electricity source. Positive side – anode Negative side- cathode

5. J.J. Thomson – cont. Cathode ray – glowing beam, which travels from the cathode to the anode. Thomson proposed that the cathode ray had a stream of small negatively charged particles. Plum Pudding Theory – Electrons are imbedded in a positive sphere. http://chemmovies.unl.edu/ChemAnime/CATHRAYD/CATHRAYD.html

6. Robert A. Millikan American Calculated by 1916 that the electron had one unit of negative charge and mass is 1/1840 the mass of Hydrogen atom.

7. Sir James Chadwick Subatomic particles with no charge – neutron. Mass is nearly equal to that of the proton.

8. Subatomic Particles Particle charge relat.mass mass Proton(p + ) + 1 1amu 1.67x10 -24 g Neutron(n 0 ) 0 1amu 1.67x10 -24 g Electron(e - ) - 1 1/1840 9.11x10 -28 g amu amu = atomic mass unit

9. Ernest Rutherford 1911 – Gold Foil Experiment Proposed that almost all of the mass and all the positive charge are concentrated in a small region at the center of the atom called the nucleus. Nucleus- the central core of the atom, composed of protons and neutrons. http://www.shsu.edu/~chm_tgc/sounds/ruther.mov

10. The Development of Atomic Models Dalton’s Atomic Model – atoms are billiard ball spheres. Thomson’s Model – Electrons are imbedded in a positive sphere. Rutherford’s Model – Positive dense tiny nucleus is surrounded by electrons in empty space.

11. Atomic Models Neils Bohr – 1913, Danish physicist Planetary Model Electrons have a fixed path.

12. Current Model – Quantum Mechanical Model Erwin Schrodinger – 1926 Quantum Mechanical Model – the modern description of the electrons in the atoms comes from the mathematical solution to the Schrodinger equation. Electron Cloud Think of a fly trapped in a jar!